Abstract:
An atomic force microscope (AFM) operating in a contact mode together with microcalorimetry experiments, (with ammonia and pyridine as probe molecules), have been used to investigate the effects of thermal and hydrothermal treatments on the acidity of a commercially available fluid cracking catalyst (FCC). Results have been compared with those generated from an equilibrium sample of the same catalyst obtained from a European refinery.

Large scale images reveal that the FCC surface is formed by platelets and platelet aggregates and that the catalyst porosity result mainly from elongated and narrow cracks between these plates. Aging decreases the FCC microporosity and changes the relative distribution of pore sizes in the mesomacro porosity range.

As a result, the sorption capacity for the probe molecules used, as well as the FCC acid site density, are drastically reduced. The acid site density and strength of the equilibrium FCC does not deviate significantly form that of the catalyst prior to MAT evaluation indicating that the steaming procedure used reduces the acidic properties of the fresh catalyst to those of the corresponding equilibrium sample.

The difference in cracking activity between two aged FCC having similar acidic properties illustrates the importance that the catalyst internal architecture and cracking sites availability have on FCC performance.